The principal objective of this research is to obtain information about the mechanisms of salt and water transport in specific portions of the mammalian nephron. Segments of nephron are removed from the kidney of rabbits and studied in vitro using a microperfusion method. The tissue is observed directly at high magnification. In this manner it is possible to correlate tubular function with the morphology of the living tissue. I will examine for evidence of a natriuretic hormone in volume expanded animals by testing the effect of "natriuretic" serum on net fluid absorption of isolated tubules. The influence of acute changes in serum electrolytes on fluid absorption of isolated proximal tubules will be examined. The role of the cyclic AMP system in the regulation of proximal tubule absorption will be evaluated in the course of studying the effects of theophylline, parathormone, vasopressin, epinephrine and prostaglandins. Factors which influence the control of cellular volume of proximal convoluted, proximal straight, cortical collecting tubules and papillary collecting ducts will also be investigated. Physical and chemical properties of isolated tubule basement membranes will be studied in vitro to determine the possible role of these structures in the pathogenesis of polycystic renal disease and nephrocalcinosis. These studies should further the understanding of how renal tubular cells and their supporting structures (basement membranes) contribute to the formation of urine in specific portions of the nephron.